Synthesis And Properties Of Fluorene-based Epoxy Resins With Flexible Side Chains

Fluorene-based epoxy resins are typical heat resistant epoxy resins. Owing to the existence of rigid fluorenyl, the glass transition temperature and char yield of fluorene-based epoxy resins are rather high. However, the cured products are generally rather bittle and hard to bear impact load. It is necessary to study the toughening modification on fluorene-based epoxy resins. Doping rubber particles into epoxy resin matrix and introducting flexible segments into main-chain of epoxy resin are the most common physical and chemical modification methods. These methods which effectively improved the toughness of fluorene-based epoxy resins also decreaced the rigidity and thermal stability of the materials decreased. In this paper, introducting flexible alkoxy side chains into fluorene-based epoxy resin monomers remarkably increased the toughness of the cured products without much sacrificing thermal properties.In this paper, three kinds of fluorenone monomers with flexible alkoxy side chains were synthesized via substitution reaction of 2,7-dihydroxy-9-fluorenone and bromoalkane(including 1-bromopropane, 1-bromohexane and I -bromododecane). The yield of the products is 92-97%. Then three kinds of bisphenol fluorene monomers with flexible alkoxy side chains were synthesized via condensation reaction of phenol and three kinds of fluorenone monomers with flexible alkoxy side chains by using concentrated sulfuric acid as catalyzer. The yield of the products is 86-92%. Finally, three kinds of fluorene-based epoxy resins with flexible alkoxy side chains were synthesized by epichlorohydrin and three kinds of bisphenol fluorene monomers with flexible alkoxy side chains by using hexadecyl trimethyl ammonium bromide as catalyzer. The yield of the products is 82-89%. The chemical structures of the products have been characterized by 1H NMR, 13C NMR and FT-IR. Thermal and mechanical properties of the cured products were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical thermal analysis(DMA) and universal testing machine.The characteristic parameters of curing processes were confirmed by the non-isothermal DSC curves. The curing processes were confirmed by extrapolated method. The curing kinetics and activation energy values of different curing systems were studied and calculated by Starink, Ozawa and Kissinger methods. Average activation energy values were obtained by calculation. With the increasing of the length of alkoxy flexible side chains, the average activation energy value of the curing system gradually increased and the curing activity of the curing system decreased. The reaction orders of different curing systems were confirmed by Crane method. The degree of curing-temprature curves were achieved by Starink integral and conversion method. The types of all curves were S, which showed good agreement with autocatalysis mechanism.With the increasing of the length of flexible alkoxy side chains, the melting point of epoxy resin monomers gradually decreased, which provided wider processing windows. With the increasing of the length of flexible alkoxy side chains, the storage modulus and the glass transition temperatur of the cured products also gradually decreased. The rigidness and heat resistance of the cured products decreased. However, the tensile strength, the elongation at break values, the fracture energy and the impact strength of the cured products gradually increased, with the length of flexible side chains increased. The introduction of flexible alkoxy side chains into fluorene-based epoxy resin monomers remarkably increased the toughness of the cured products.